Help Please, my fish are dying

Alright two days ago I found three dead cardinal tetras. Not to worried as I added five a week ago without a quarantine (I know I get what I deserve!, but I observed them at the store for over two weeks. Yesterday lost four more (UGH more than just lack of acclimation) and one of my Corys. Today lost one more cardinal and one cory. Started treating with Melafix yesterday. One more of my corys looks bloated on his belly and not very active. None of the fish are eating much. Black neons all look good, Bolivian Rams also look OK although not eating much. One cardinal look like white covering on eye and is swimming more alone, another cardinal sometimes swimming at a 45 degree angle and hangin out near top. I am freaking out that this is either Neon Tetra disease or worse yet TB!

I did big water change (25%) on day one of episode. Today Nitrate and Ammonia are zero, I did not check them before water change. pH is High (more than any of my scales read, but cory's have been in tank for over a month doing fine till now) and the water is VERY hard also.

The corys all lost their coloring and turned pale a couple of days ago but it came back today. The cardinal with the white slightly puffy eye is the first specific symptom I have seen.

What do you think NTD or TB or something else

Another dead cardinal this morning, his stomach looked bloated a bit. Another one swimming sometimes at a 45 degree angle. Would like to try something besides Melafix if anyone thinks that may be helpful and not just stress them more, anyone got any recommendations? The cory that looked a bit ill yesterday is still alive and seems a little more active.

30 gallon set up forever, switched over substrate removed danio's that were there forever and added african driftwood and plants about 6 weeks ago. Started slowly stocking with 5 Black neon tetras, 2 Bolivan rams, 3 ottos (1 died), 5 Cory's (were in tank for last 4 weeks fine, just started losing them), and 9 cardinal tetras that are now down to 3. Also two Amano shrimp (still doing fine). Penquin 350 with two rite size C filters (up to 70 gallons rated) only change one at a time and not at same time as water changes. Planted with Java fern, Anabias, and just recently subwassertang. Quickcrete premium playsand at depth of less than 1 inch. The sand is silica. pH max out both test strips and liquid test kit (7.6 highestest rating on liquid kit, 8 on strips). Nitraites and ammonia don't read anything. Nitrates about 40 ppm. GH and KH both at 300 ppm read (max of kits). Note latest dead fish did not have anything over eyes like the last one.

Note after 1st fish died, I did a water change with 3 to 1 ratio RO water to tap. pH of mix was 7.4 and GH was 100 ppm. I used 6 gallons of this (20% water change) over about 4 hours so as to make the change gradual - 3 gallons added then 4 hours later another 3 gallons. Could this have increased my problems. Yesterday I also replaced one filter cartridge but left in charcoal, even though Melfix dosing instructions suggested I take it out.

Cardinals were tank raised from LFS. The cory is Agassizi I am guessing or something similar (in looks). It was from Petco and was mislabeled. My otto's are doing fine (as far as I can tell). Any medication's that won't kill my shrimp that might help?